Interactions of Neisseria gonorrhoea with human neutrophils and serum were studied to delineate possible pathogenic mechanisms. Studies were carried out to (A) characterize PMN non-opsonic recognition of Opa+ gonococci and (B) to try to identify other gonococcal interactions with host components which may be mediated by Opa proteins. (A) Neutrophil non-opsonic recognition of Opa+ gonococci was investigated. (1) Earlier studies using oligopeptides to test for neutrophil recognition of the Opacity protein fourth loop sequence were expanded to include an additional scrambled control peptide with the identical amino acid composition as the normal sequence peptide. As before, the control peptide showed neither a dose-dependent inhibition of PMN recognition of Opa+ gonococci or direct stimulation of Pbs when coated on surfaces while the normal sequence peptide did both. These findings suggest a role for the Opa protein fourth loop in gonococcal-neutrophil interactions. (2) Monoclonal antibodies against PMN surface proteins have been utilized to begin to characterize PMN surface receptors which may be mediating recognition of Opa+ gonococci. Preliminary results indicate that the PMN CL response to Opa+ gonococci is mediated, at least in part, by the integrin receptor family. (B) Studies of gonococci exposed to normal human serum have demonstrated that Opa+ gonococci show relatively more serum resistance than Opa- gonococci when the bacteria express the lipooligosaccharide type B phenotype. All gonococci expressing lipooligosaccharide type A phenotype demonstrate an extremely high level of serum resistance, resistant to killing by greater than 75% normal serum.